Update instructions for rich text custom rendering [ci-skip]

[jonathanhefner]

Since #43110, we no longer generate .scss files by default.

This commit:

• Changes remaining .scss filenames to .css.
• Updates the instructions for omitting the default Trix styles.
• Eliminates repetitive use of the word "default".
• Reorders the content slightly for better flow.

[p8]

Looks great!

[seanpdoyle]

@jonathanhefner @p8 with the removal of this line, the Action Text Overview only mentions "sanitization" in one place:

rails/guides/source/action_text_overview.md

Lines 95 to 99 in 00dfa10

	And finally, display the sanitized rich text on a page:
	```erb
	<%= @message.content %>
	```

Surprisingly, neither ActionText::RichText nor ActionText::Content mention anything about sanitizing content in their RDoc comments.

Securing Rails Applications doesn't mention Action Text at all, nor does it mention that it's is serialized by rails-html-sanitizer.

Was the removal of this line intentional, or accidental? Where would be the most appropriate place to improve the Action Text documentation with regard to its security implications?

[seanpdoyle]

I've opened #49696 to try and expand upon sanitization support.